Hi Pilot08
welcome to the Apple Community.
re: Why is my phone changing who receives certain texts
You can try restarting (turn off, turn on) your iPhone if it is not working as expected.
And
Try looking in your "contacts" to check if the phone / addresses of the two people concerned are actually correct there.
Else:
You might want to browse through this trouble-shooting Apple Support article to see if anything there helps you:
If you can't send or receive messages on your iPhone or iPad - Apple Support
__________________________________________________________________________________
Also: If you still need help:
You might want to have a look through your Message settings to check all is in order there.
Eg: An unwanted setting may have been accidentally changed / triggered.
eg: Settings > Messages > Send and Receive
Settings > Messages > Text Message Forwarding
_________________________________________________________________________________-
If needed:
There is a "Get Support" link at the bottom of the Messages Support Team's Website (link below)
Messages for iPhone, iPad, Apple Watch, and Mac - Official Apple Support
All the best :-)